1. Technical Field
The present invention relates to a vehicular lighting device and a beam welding method, and in particular to a technology for reliably joining the housing of a vehicular lighting device and a transparent cover by way of beam welding.
2. Related Art
In the manufacture of a vehicular lighting device, for example an automobile lighting device, a process is required to mount a transparent cover generally called a lens onto a housing so as to cover its front opening.
In the related art, a method was generally used for protruding seal legs on the peripheral edge of a transparent cover, forming a groove that receives the seal legs on a housing, and receiving the seal legs of the transparent cover by the groove of the housing via a sealant or an adhesive in order to mount the transparent cover onto the housing. In this mounting method, the seal legs and the groove are provided at a position protruded outward from the peripheral edge of the transparent cover or housing. When the transparent cover is viewed from the front, its peripheral edge includes a portion occupying a relatively large area that does not transmit light, or a dark portion. This results in a smaller light-emitting area with respect to the entire area of the transparent cover.
One solution to this problem was a method for directly joining the seal legs of the transparent cover to the joining surface of the housing. By directly joining the seal legs of the transparent cover to the housing, the resulting joining margin has a very small width and the dark portion of the peripheral edge of the transparent cover occupies a very narrow width. The direct joining means has been generally hot plate welding or vibration welding. For such means, the melted materials of the transparent cover and the housing are mixed and then cooled and solidified at the joining part of the transparent cover and the housing. The drawback of this method is that excessive molten material oozes from the joining part, which impairs the external appearance as seen through the transparent cover from the front.
This method also has restrictions on the shape. These include: the tilt angle is limited; an almost flat surface is required in the direction of vibration in vibration welding; and excessive tilting is not allowed with respect to the direction the hot plate is applied in hot plate welding. As such, this approach cannot meet today's need for three-dimensional designs.
Another method joins the transparent cover and the housing via laser welding. In laser welding, by controlling the laser output and the spot diameter at the joining part as well as the scan speed, the melting state of the material, that is, the irradiation energy at the joining part can be controlled to a certain degree. This prevents oozing of the molten material caused by excessive melting as well as supports a complicated shape. By irradiating laser beams onto the housing through the transparent cover, the housing is heated and melted, which transfers heat to the transparent cover to cause it to melt, thus compatibilizing and welding them both.
In the beam welding such as laser welding, insecure contact between the welding part of the transparent cover and that of the housing will result in poor joining.
The transparent cover and the housing of a vehicular lighting device are resin molded items. Influenced by curling or deformation that follows molding, the intended shape is difficult to obtain in reality. Thus, it is difficult to keep the transparent cover in close contact with the housing along the entire circumference. Poor contact between the transparent cover and the housing prevents heat given to the housing by irradiation of laser beams from being transferred to the transparent cover, thus disabling the welding process.